os/user: clean error message formatting

Change-Id: I02728c690a377ecdd2a6bc92d1606cbae3e2723a
Reviewed-on: https://go-review.googlesource.com/c/go/+/196677Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/user/lookup_stubs.go

@@ -72,7 +72,7 @@ func current() (*User, error) {
 
 func listGroups(*User) ([]string, error) {
 	if runtime.GOOS == "android" || runtime.GOOS == "aix" {
-		return nil, errors.New(fmt.Sprintf("user: GroupIds not implemented on %s", runtime.GOOS))
+		return nil, fmt.Errorf("user: GroupIds not implemented on %s", runtime.GOOS)
 	}
 	return nil, errors.New("user: GroupIds requires cgo")
 }